Swashbuckle/Swagger + ASP.Net Core: "Failed to load API definition"

I develop an ASP.NET Core 2 application and included Swagger. Everything worked fine until I introduced a method without explicitly defining the HTTP action:

public class ErrorController : Controller
{
    [Route("/error")]
    public IActionResult Index()
    {
        return StatusCode(500, new Error("Internal error."));
    }
}

When I started the app with this method, the following message showed up:

> Failed to load API definition. > > Errors
> Fetch error Internal Server Error /swagger/v1/swagger.json

As soon as I explicitly set e.g. [HttpGet] the error disappears. The problem with this is, I need this method to fire for all possible HTTP operations. Of course, I could specify all operations explicitly, but I have the feeling Swagger should be able to handle this correctly.

Why does Swagger behave this way?

Is there any configuration I can use?

Add Httpxxx([HttpGet], [HttpPost], ...) attribute for each Action method, or [ApiExplorerSettings(IgnoreApi = true)]

Simply you can look into the log in the Output window. The actual error can be seen there in my case, I missed adding HTTP action on top of a methods

The option ResolveConflictingActions should be working on this case...

Here is the actual error:

System.NotSupportedException: Ambiguous HTTP method for action

That is coming from: https://github.com/domaindrivendev/Swashbuckle.AspNetCore/blob/86cc761bc4f5dda796f80ad8dfd8bc205542a6f6/src/Swashbuckle.AspNetCore.SwaggerGen/Generator/SwaggerGenerator.cs#L90

I think this is a bug, if you are truly interested you should report it to the project

Instead of blindy guessing what could be the issue, navigate to

http:///swagger/v1/swagger.json

In my case this could have been resolved by using the c.CustomSchemaIds(x => x.FullName);

which is a horrible workaround, but might be a quick fix for someone in need. My solution was to rename and clarify the path for those endpoints

I don't know if this has been resolved or not but one way you can go about this is by decorating the method with:

[ApiExplorerSettings(IgnoreApi = true)]

This will ensure that the method in question is ignored by Swagger.

Another possible issue is that the endpoint needs to be complete from the domain root.

I had:

app.UseSwaggerUI(c =>
{
     c.SwaggerEndpoint("/swagger/v1/swagger.json", "V1 Docs");
});

I had to use:

 app.UseSwaggerUI(c=>
{
     c.SwaggerEndpoint("/myApi/swagger/v1/swagger.json", "V1 Docs");
    
});

In ASP.NET Core, if there is a controller endpoint like:

[Route("images")]
[HttpGet("{id}")]

This can also fail with fetch failed. The solution is to have something like

[HttpGet("images/{id}")]

Same thing goes for HttpPost.

In addition to Helder Sepulvedas answer and from 'Monte-Christos' answer in this github issue - [Actions require unique method/path combination for Swagger][1] > I found the place to configure ResolveConflictingActions in ASP.NET > Core apps. In your Setup class, add this to the ConfigureServices() > method:

services.AddSwaggerGen(c => 
  { 
    other configs...;
    c.ResolveConflictingActions(apiDescriptions => apiDescriptions.First());
  });

This done the trick for me! [1]: https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/390

Swagger also throws the same exception if there are public methods that are not actions in a controller. The fix is to make all of them protected or private or as mentioned add the attribute [ApiExplorerSettings(IgnoreApi = true)].

in my case i use this code as like as .net code

[ActionName("Login")]
[HttpPost]

now i change it for use on net core web api

[HttpPost("Login")]

And it works right

I was also getting this error because i created a controller which dosent have [Route("api/[controller]")]. After putting it, the error went away.

app.UseSwagger()
   .UseSwaggerUI(c =>
   {
       c.SwaggerEndpoint("/swagger/v1/swagger.json", "DOA.API V1");
   });

What worked for me is adding [NonAction] attribute to the public methods that are not API calls in my controller wrapper.

I had the same issue. In my case, All of my controllers inherited from a BaseController. in this base class, I got a public action which returns UserId according to Claims. I set the [NonAction] attribute on the top of this action.

[ApiController]
[ApiResultFilter]
[Route("api/[controller]")]
[Authorize(AuthenticationSchemes = JwtBearerDefaults.AuthenticationScheme)]

public class BaseController : ControllerBase
{

    [NonAction]
    public int GetCurrentUserId()
    {
        return int.Parse(this.User.Claims.First(p => p.Type == ClaimTypes.NameIdentifier).Value);
    }
}

Double check , if you have used same url name in your same controller. It was happened to my code

I also had this problem. I checked and applied all of the solutions for swagger config but the issue still remained. Finally, I checked the output panel and the problem was because of [DefaultValue("SYSDATETIMEOFFSET()")].

The answer is here: Check the output panel and you will find the answer

Guess what... started getting this error page "Failed to load API definition":

By looking at the app's console or the output window in Visual Studio it told me the following:

I added a public method called Client to the Web API BaseController and Swagger was thinking it should be included in the API.

To solve the problem: mark that method as protected:

I was getting a TypeLoadException on a class that I was deleting that was unused. The fix in my case was to delete the bin/obj/Debug folder contents. Clean solution + rebuild solution did not fix for me.

My error reason was that same url name,

 [HttpGet("get/LeaveCommand/{id}")]

I use same url and swagger cannot get them

 [HttpGet("get/LeaveCommand/{id}")]

It happened because of Newtonsoft.Json in my case. But the thing is I was not using it. One of the packages may be depend on it but I did not have time to check.

So just check the output panenl solve the related issue.

If you have in your models (request or response) properties of type which inherits/implements types like System.ComponentModel (or other types) this will throw an error

"The JSON property 'item' is defined multiple times on type"...

Try to ignore this property using [JsonIgnore] attribute of Newtonsoft.Json

In my case i had a getter of type DataTable

In the Startup file you need to make sure that you add

services.AddSwaggerDocument();

before you add

app.UseOpenApi();
app.UseSwaggerUi3();

or it can result in this error

> Fetch error undefined /swagger/{documentName}/swagger.json

for core 3 I had the same issue and was really confused that problem was in slash.

Configuration was:

services.AddSwaggerGen(c =>
			{
				c.SwaggerDoc("v1", new OpenApiInfo { Title = "my-API", Version = "v1" });
			});

This swagger endpoint threw the message of TS:

app.UseSwaggerUI(c =>
			{
				c.SwaggerEndpoint("/v1/swagger.json", "my-API v1");
			});

And finally I got it worked with removing the first slash in the URL:

app.UseSwaggerUI(c =>
			{
				c.SwaggerEndpoint("v1/swagger.json", "my-API v1");
			});

For me the problem was having registering swagger after MapControllers middleware. Once I moved before it worked.

For me it was a very simple issue, i had 3 [HttpGet] methods which turned out i needed to change the "createOrder" method to a [HttpPost]

        [HttpGet]
        public async Task<ActionResult<List<Order>>> GetOrders()
        {
            return await _context.Orders.Include(o => o.OrderItems).Where(x => x.BuyerId == User.Identity.Name).ToListAsync();
        }

        [HttpGet("{id}", Name = "GetOrder")]
        public async Task<ActionResult<Order>> GetOrder(int id)
        {
            return await _context.Orders.Include(x => x.OrderItems).Where(x => x.BuyerId == User.Identity.Name && x.Id == id).FirstOrDefaultAsync();
        }

        [HttpPost]
        public async Task<ActionResult<int>> CreateOrder(CreateOrderDto orderDto)
        {
            var basket = await _context.Baskets.RetrieveBasketWithItems(User.Identity.Name).FirstOrDefaultAsync();
            var items = new List<OrderItem>();
}

For me it was solved by adding the HttpGet attribute and removing the index methods.

The issue for me was caused by lazy/frustrated refactoring and I was able to determine the issue by reading the debug console when running the API in debugger mode.

Due to the bad refactoring, I ended up with two models with the same name, and Swagger was getting confused.

I had something like:

PROJECT
├───Auth
│       AutheController.cs
│       UserDto.cs
│
└───Controllers
    │   MyContrller.cs
    │
    └───Models
            UserDto.cs

Having two UserDto models is what was confusing Swagger. Cleaning this up fixed the issues.